Maternal phenylketonuria (PKU) is a major unresolved problem in PKU, threatening to produce microcephaly, mental retardation, congenital heart disease, dysmorphic facies, and low birth weight in many offspring throughout the world. Phenylalanine restricted dietary therapy during pregnancy may offer some degree of fetal protection, but the extent of and conditions for this protection are not yet known. The Maternal PKU Collaborative Study was developed in 1984 to address these questions. The objectives of this study are to determine: 1) Does the phenylalanine restricted diet reduce the frequency of mental retardation, spontaneous abortion, low birth weight, congenital malformations, neurological and behavioral impairment reported in pregnancies of hyperphenylalanine (HPA) mothers who were on unrestricted phenylalanine intake during pregnancy? 2) Is pregnancy outcome in HPA women who restrict phenylalanine intake during pregnancy comparable to that of non-HPA women? 3) Is pregnancy outcome in HPA women related to maternal phenylalanine levels during pregnancy. 4) Is gestational age at the onset of intervention predictive of fetal outcome? 5) Are there beneficial effects of starting diet prior to conception? 6) What are the levels of tyrosine and trace elements during pregnancy and what are the effects on pregnancy outcome of supplementation if levels are found to be reduced?